HIGH ACCUMULATION RATES AND THE GENERATION OF THICK PALOUSE LOESS VIA TOPOGRAPHIC TRAPS, JUNIPER CANYON, OR

The late-Pleistocene to Holocene L1 loess unit at Juniper Canyon, OR is the thickest accumulation of L1 loess yet recorded in the Palouse region of the Pacific Northwest. The 7.6 m of L1 loess lies immediately downwind of Juniper Canyon, which runs perpendicular to prevailing southwesterly winds and is incised >200 m into underlying bedrock. A model proposed on topographic effects of the thickness and distribution of loess previously conceived and applied to the Great Plains (Mason et al., Geomorph. 1999, v. 28) was tested at Juniper Canyon, where loess thickness has been accentuated via topographic trapping of eolian sand. The topographic trap model also applies to other areas of the Palouse including Eureka Flat, and can help explain significant differences in thickness of near-source accumulations of loess. Deflation of sand- and silt-rich late-Wisconsin glacial outburst-flood deposits upwind of Juniper Canyon form sand dunes and generate dust for downwind suspension transport. The sand dunes migrate to the upwind edge and avalanche into Juniper Canyon, removing the saltation-dominated bed load from the eolian system. Downwind of the canyon, thick loess has accumulated, protected from saltating sand grains. Grain-size distribution is very uniform throughout the entire 7.6 m of L1 loess, with a mean size of 50 microns, illustrating the effectiveness of the canyon at trapping the coarser load. East and west of the zone where dunes intersect the canyon, L1 loess is 4 m thick or less, demonstrating that the topographic trap accentuates loess thickness. The Mt. St. Helens set 'S' tephra (15 ka) at the base of L1 loess at Juniper Canyon allows the calculation of an average accumulation rate of about 0.5m/ka, nearly double the rate at any other measured section of L1. Upwind of Juniper Canyon, the older L2 loess, capped by the Washtucna paleosol, is exposed in blowouts, suggesting the canyon may not have been acting as a trap for dune sand before 15 ka, and that dust-deflating saltating sands were perhaps restricted further upwind in the Umatilla Basin. The Umatilla Basin contains widespread fine-grained flood deposits upon which dunes and sand sheets migrate, suggesting the basin is a major source for loess at Juniper Canyon and further downwind in the Palouse.